Field
The present invention relates to a power amplifier circuit.
Description of Related Art
A power amplifier circuit mounted in mobile communication equipment, such as a cellular phone, usually uses a bipolar transistor as an amplifier. The bipolar transistor has a thermal positive feedback characteristic, in which, as the temperature of a transistor device increases, a collector current increases, causing the temperature to further rise with a resultant increase in the collector current. Hence, if, for example, a bipolar transistor is composed of a plurality of unit transistors, then the occurrence of temperature differences among the unit transistors causes a unit transistor having a relatively high temperature to undesirably draw in the collector currents of other unit transistors. This may consequently lead to the occurrence of a phenomenon known as a current collapse phenomenon, in which the collector current of the entire transistor group composed of the plurality of unit transistors decreases.
There has been known, therefore, a configuration in which, for example, a resistive element (hereinafter referred to also as “the emitter ballast resistor”) is inserted between the emitter of each unit transistor and ground to restrain an increase in temperature of each unit transistor. The insertion of the emitter ballast resistor makes it possible to restrain uneven temperature distribution among the plurality of unit transistors, thus avoiding the occurrence of the current collapse phenomenon.
The effect of such an emitter ballast resistor to restrain uneven temperature distribution decreases unless the emitter ballast resistor has a resistance value of a certain level or more. However, if the resistance value of the emitter ballast resistor is excessively large, then the output power of the power amplifier circuit may decrease or power added efficiency may deteriorate.